Friction - Exemplar Solutions 1

CBSE Class 8 Science
NCERT Exemplar Solutions

1. Whenever the surfaces in contact tend to move or move with respect to each other, the force of friction comes into play,
(a) only if the objects are solid.
(b) only if one of the two objects is liquid.
(c) only if one of the two objects is gaseous.
(d) irrespective of whether the objects are solid, liquid or gaseous.
Ans: (d) irrespective of whether the objects are solid, liquid or gaseous.
Explanation: When two surfaces in contact "tend to move" or "move with respect to each other", the force of friction ("static friction" or "sliding friction" respectively) comes into play irrespective of whether the surfaces are solids and/or fluids (liquids or gases)

2. In Fig.12.1, a boy is shown pushing the box from right to left. The force of friction will act on the box
(a) from right to left ()
(b) from left to right ()
(c) vertically downwards ()
(d) vertically upwards ()
Ans: (b) from left to right ()
Explanation: The force of friction always acts in the direction opposite to the direction of motion. So here the force of friction will act from left to right ()

3. To sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable?
(a) stone
(b) plastic block
(c) wooden block
(d) glass block
Ans: (a) stone 
Explanation: Hard and rough surface like stone will generate more friction as compared to the friction generated by other given options. Hence, a stone will sharpen the blade most efficiently.

4. A toy car released with the same initial speed will travel farthest on:
(a) muddy surface
(b) polished marble surface
(c) cemented surface
(d) brick surface
Ans: (c) cemented surface
Explanation: The frictional force between a polished marble surface and the ball will be the least. A toy car released on a cemented surface will travel the farthest.

5. If we apply oil on door hinges, the friction will:
(a) increase.
(b) decrease.
(c) disappear altogether.
(d) will remain unchanged.
Ans: (b) decrease
Explanation: When oil is applied to door hinges, the friction will decrease due to the smoothness of the hinge surface.

6. Which of the following statements is incorrect?
(a) Friction acts on a ball rolling along the ground.
(b) Friction acts on a boat moving on water.
(c) Friction acts on a bicycle moving on a smooth road.
(d) Friction does not act on a ball moving through air.
Ans: (d) Friction does not act on a ball moving through air.
Explanation: The force of friction comes into play between two surfaces irrespective of whether the two surfaces are solids and/or fluids (liquids or gases). The frictional force exerted by fluids is called "drag". Hence, when a ball is moving through air, it experiences a drag.

7. A boy rolls a rubber ball on a wooden surface. The ball travels a short distance before coming to rest. To make the same ball travel longer distance before coming to rest, he may:
(a) spread a carpet on the wooden surface.
(b) cover the ball with a piece of cloth.
(c) sprinkle talcum powder on the wooden surface.
(d) sprinkle sand on the wooden surface.
Ans: (c) sprinkle talcum powder on the wooden surface.
Explanation: Sprinkling talcum powder makes the wooden surface smooth and hence the wooden surface exerts lesser friction on the moving ball. This is the reason why people spread talcum powder on the carom-board before playing to make the surface smooth.

8. In a large commercial complex, there are four ways to reach the main road. One of the path has loose soil, the second is laid with polished marble, the third is laid with bricks and the fourth has gravel surface. It is raining heavily and Paheli wishes to reach the main road. The path on which she is least likely to slip is:
(a) loose soil.
(b) polished marble.
(c) bricks.
(d) gravel.
Ans: (d) gravel
Explanation: Gravel will make the surface rough and will increase the force of friction. Hence, Paheli is least likely to slip on the path laid with gravel.